In the document FR-A-2 572 899, it is known to produce a food product which is either expanded or not, filled or not, such as bread, a brioche-like product or the like, from a material containing carbohydrates, using an extrusion cooking method.
In such a method, a screw of an extruding machine is driven in rotation in a cylindrical body and carries out an appropriate treatment on the raw material which comprises, in particular, at least one kneading step, one compression step and one cooking step of the raw material. The downstream end of the extrusion cylinder is provided with an extrusion die, through which the food product produced from the raw material emerges from said extruding machine.
Moreover, food products are known which are produced from carbohydrates and contain inclusions, such as pieces of nut or chocolate or other pieces. Said food products are produced by using traditional methods, in a first step of which a dough is made from various ingredients and kneaded in a vat. The inclusions are introduced into the dough during this first step, after which the dough is removed from the vat, divided up into pieces and placed in a cooking oven. A second step consists in cooking the pieces of dough in this oven.
This type of traditional method has drawbacks which principally lie in the fact that it is of long duration and consumes a high amount of energy, which is translated into an increased cost for producing the food product. Moreover, with these traditional methods, it is difficult to obtain homogenous food products due to production conditions. Moreover, said methods do not allow products with expanded and crisp textures to be produced, such as those obtained by the extrusion cooking of starch and protein-based products.
More specifically, obtaining expanded food products containing visible inclusions by means of extrusion cooking poses problems.
More specifically, to produce an expanded food product it is necessary for the product, which is extruded at the outlet of the extruding machine and which is formed by a mixture containing water, to pass from a zone, in the pressure-temperature diagram of water, where the water is in the liquid state to a zone where normal pressure and temperature conditions prevail and where the water is in the gaseous state.
The conditions prevailing inside the cylinder of the extruding machine are exacerbated by high shear rates, due to the co-penetrating and self-cleaning screws of said extruding machine, as the material has to be transformed or cooked by the application of mechanical energy. To achieve this, the composition of the screws of the extruding machine conventionally comprises high shear mixing zones, with screw elements of reverse pitch or kneading discs, which mill and considerably reduce the size of the inclusions introduced at the inlet in the region of the supply zone of the extruding machine.
Moreover, raised pressure levels also prevail inside the cylinder of the extruding machine in order to create zones which are completely filled where the transformation of the material takes place.
Finally, the construction of the extrusion dies located at the outlet of extruding machines is not generally adapted to the flow of mixtures enriched with macroscopic inclusions which may create obstructions.
As a result, the introduction of inclusions into the cylinder of the extruding machine in the region of the zone for supplying raw material causes the degradation of these inclusions as they are subjected to excessive crushing. Thus, the presence of these inclusions in the final product is not visible and said inclusions are degraded.
To resolve this problem, the person skilled in the art naturally considers introducing inclusions into a zone located downstream of the supply zone relative to the direction of flow of material in this extruding machine. However, this does not resolve the problem as the inclusions also have to be introduced into the cylinder at low pressure and at low shear, to avoid the mechanical crushing thereof.
Moreover, in the extruding machine, the worked product is cooked and brought to a temperature which is equal to, or slightly greater than, 100° C. If the inclusions are introduced into this product in a zone at low pressure, the overheated water which it contains is released in the form of steam, preventing the inclusions from entering the product.
In the document FR-A 2 879 898, a method is also known for obtaining a food product from at least one material containing carbohydrates and/or proteins by means of an extruding machine having at least one screw. This product contains solid inclusions larger than 0.5 mm, introduced continuously into the material as soon as possible during the cooking of said material during a compression, kneading and cooking step.
The inclusions are introduced into the cylinder of the extruding machine using a metering device comprising a feed duct for said inclusions which discharges into said cylinder in the region of the cooking zone.
This known method has drawbacks.
More specifically, during the working and heating of the material in the cylinder of the machine, steam is produced which naturally rises into the metering device through the feed duct. This rising steam tends to moisten the inclusions before they are fed into the cylinder and tends to interfere with the regular metering of said inclusions into the material.
Moreover, given the incorporation of the inclusions in the cooking and intense mixing zone, said inclusions are subjected to relatively harsh conditions due to the shear force and they are also subjected to a raised temperature of approximately 130 to 150° C.
Taking account of these conditions and added to the fact that they are pre-moistened due to the steam, more fragile inclusions, such as cereal flakes, dried red fruits or low-fibre vegetables, dissolve and become buried in the mass of cooked material and disappear. The yield of visible inclusions in the final product is practically zero.
Inclusions which are slightly fragile, such as fibrous vegetables, are partially degraded and provide a yield of 10 to 20% of visible inclusions in the final product.
Thus, only inclusions which are very rich in fibre, such as small grains, and above all very hard inclusions, such as broken cereals, resist this degradation after their introduction into the extruding machine and provide a yield of 20 to 50% of visible inclusions in the final product. This result is acceptable but costly as it requires a relatively high consumption of inclusions to obtain a satisfactory result as regards visibility of inclusions in the product obtained.
Thus, the incorporation of inclusions in an expanded and edible food product obtained by extrusion cooking poses problems.